Which of the following properties is responsible for the development of colour to the colloids?

Identify the factor responsible for colour of transition metal compounds.

Identify the number of unpaired electrons present and geometry respectively of $[\text{Co(NH}_3)_6]^{3+}$ complex.

Identify oxidation state of cobalt ion in the complex, $[\text{Co(NH}_3)_5\text{Br}]\text{SO}_4$ .